U.S. patent number 10,795,036 [Application Number 15/313,101] was granted by the patent office on 2020-10-06 for gamma-ray imaging.
This patent grant is currently assigned to Australian Nuclear Science and Technology Organisation. The grantee listed for this patent is Australian Nuclear Science and Technology Organisation. Invention is credited to David Boardman, Alison Flynn, Dale Prokopovich, Adam Sarbutt.
United States Patent |
10,795,036 |
Boardman , et al. |
October 6, 2020 |
Gamma-ray imaging
Abstract
A coded mask apparatus is provided for gamma rays. The apparatus
uses nested masks, at least one of which rotates relative to the
other.
Inventors: |
Boardman; David (Lucas Heights,
AU), Sarbutt; Adam (Lucas Heights, AU),
Flynn; Alison (Lucas Heights, AU), Prokopovich;
Dale (Lucas Heights, AU) |
Applicant: |
Name |
City |
State |
Country |
Type |
Australian Nuclear Science and Technology Organisation |
Lucas Heights, NSW |
N/A |
AU |
|
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Assignee: |
Australian Nuclear Science and
Technology Organisation (Lucas Heights, NSW,
AU)
|
Family
ID: |
1000005097040 |
Appl.
No.: |
15/313,101 |
Filed: |
May 22, 2015 |
PCT
Filed: |
May 22, 2015 |
PCT No.: |
PCT/AU2015/000302 |
371(c)(1),(2),(4) Date: |
November 21, 2016 |
PCT
Pub. No.: |
WO2015/176115 |
PCT
Pub. Date: |
November 26, 2015 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170322327 A1 |
Nov 9, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
May 22, 2014 [AU] |
|
|
2014901905 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G21K
1/10 (20130101); G01T 1/161 (20130101); G01T
1/167 (20130101); G01T 1/295 (20130101); G21K
1/02 (20130101); G01V 5/0016 (20130101) |
Current International
Class: |
G01T
1/29 (20060101); G01T 1/167 (20060101); G01V
5/00 (20060101); G21K 1/10 (20060101); G21K
1/02 (20060101); G01T 1/161 (20060101) |
Field of
Search: |
;250/370.09 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
JL. Starck et al., "Compressed Sensing in Astromony," retrieved
from the internet:
url:http://convexoptimization.com/TOOLS/CompressSensingAstro.pd- f,
38 pages (2008). cited by applicant .
J. Bobin et al. "Compressed Sensing in Astronomy", IEEE J. Sel.
Topic in Sig. Proc. 2(5) (2008) 718-726. cited by applicant .
J. L. Starck et al: "Compressed Sensing in Astronomy", Jul. 22,
2008, pp. 1-38, (May 30,2019,11:20 AM)
https://convexoptimization.com/TOOLS/CompressSensingAstro.pdf.
cited by applicant .
Shen, et al., "Spinning disk for compressive imaging," Optic
Letters, vol. 37, No. 1, Jan. 1, 2002 (3 pages). cited by applicant
.
G.K. Skinner: "Coded mask imagers when to use them--and when not,"
New Astronomy Reviews, vol. 48 No. 1-4, Feb. 1, 2004, pp. 205-208.
cited by applicant.
|
Primary Examiner: Porta; David P
Assistant Examiner: Gutierrez; Gisselle M
Attorney, Agent or Firm: Ball; Jonathan D. Greenberg
Traurig, LLP
Claims
What is claimed is:
1. A mask apparatus for use in compressed sensing of incoming
radiation, comprising: two or more coded masks having a body
portion comprised of a material that modulates the intensity of the
incoming radiation; wherein each of said masks has a plurality of
mask aperture regions, that allow a higher transmission of the
radiation relative to said body portion, the higher transmission
being sufficient to allow reconstruction of compressed sensing
measurements; wherein said coded masks are nested; and at least two
of said coded masks are configured to rotate relative to one
another.
2. The mask apparatus of claim 1, wherein: the coded masks are
cylindrical.
3. The mask apparatus of claim 1, wherein: each of the coded masks
has a top and a bottom, and the mask apparatus further comprises a
radiation shield that modulates the intensity of the incoming
radiation and that covers the top and bottom of the coded
masks.
4. The mask apparatus of claim 1, wherein: each of the coded are
hemispherical, segments of spheres, or spherical.
5. The mask apparatus of claim 1, wherein: the plurality of mask
aperture regions of each of the coded masks are equal in number to
a power of two.
6. The mask apparatus of claim 1, wherein: each of the coded masks
is formed from a material selected from the group consisting of:
tungsten, lead, gold, tantalum, hafnium and their alloys.
7. The mask apparatus of claim 1, wherein each of the coded masks
is formed: i) from a material that modulates incoming gamma-ray
radiation; ii) from a material that modulates incoming optical or
infrared radiation; iii) formed from a material that modulates
incoming neutron radiation; or iv) from a material that modulates
both incoming gamma-ray radiation and neutrons.
8. The mask apparatus of claim 7, wherein: some of the mask
aperture regions are modulating regions for gamma-rays and some of
the mask aperture regions are modulating regions for neutrons.
9. The mask apparatus of claim 1, wherein the coded masks are
concentric.
10. The mask apparatus of claim 1, wherein the mask apparatus has
two coded masks, wherein the two coded masks are configured to be
rotated relative to one another.
11. The mask apparatus of claim 2, wherein the mask apparatus has a
horizontal field of view of 360.degree..
12. The mask apparatus of claim 1, wherein the coded masks are
hemispherical and the mask apparatus has a field of view of 2.pi.
or coded masks are spherical and the mask apparatus has a field of
view of nearly 4.pi..
13. The mask apparatus of claim 1, further comprising a radiation
shield formed of a material that modulates the intensity of the
incoming radiation and that surrounds the coded masks; wherein the
radiation shield has an opening that limits a field of view of a
radiation sensor located within the one or more coded masks.
14. The mask apparatus as claimed in claim 13, wherein the
radiation shield is cylindrical.
15. The mask apparatus as claimed in claim 14, wherein the
radiation shield has an arcuate opening that limits the field of
view of the radiation sensor to an arc defined by the opening.
16. The mask apparatus of claim 13, wherein each of the coded masks
has a top and a bottom, and the mask apparatus further comprises a
further radiation shield that modulates the intensity of the
incoming radiation and that covers the top and bottom of the coded
masks.
17. The mask apparatus according to claim 1, wherein said
modulation comprises attenuating or scattering said incoming
radiation.
18. The mask apparatus of claim 1, wherein at least one of said
coded masks is arcuate, cylindrical, hemispherical, segments of
spheres, or spherical.
19. A radiation detection method, comprising: making compressed
sensing measurements of radiation from one or more radiation
sources with at least one radiation sensor and a mask apparatus,
the mask apparatus comprising: two or more coded masks having a
body portion comprised of a material that modulates the intensity
of incoming radiation, each of said one or more masks having a
plurality of mask aperture regions that allow a higher transmission
of the radiation relative to said body portion, the higher
transmission being sufficient to allow reconstruction of compressed
sensing measurements; wherein one or more of the coded masks is
configured to rotate; and at least two of the coded masks are
configured to rotate relative to one another; wherein the incoming
radiation from the one or more radiation sources passes through the
coded masks before detection by the at least one radiation
sensor.
20. A method of decommissioning, decontamination, environmental
monitoring, medical imaging, astronomy or security, comprising a
radiation detection method as claimed in claim 19.
21. A radiation detection method as claimed in claim 19, wherein
each of the one or more masks is arcuate, cylindrical,
hemispherical, segments of spheres, or spherical.
22. A compressed sensing radiation imager, comprising: at least one
radiation sensor located within a mask apparatus that comprises:
two or more coded masks having a body portion comprised of a
material that modulates the intensity of incoming radiation, each
of said two or more coded masks having a plurality of mask aperture
regions that allow a higher transmission of the radiation relative
to said body portion, the higher transmission being sufficient to
allow reconstruction of compressed sensing measurements; wherein
one or more of the coded masks is configured to rotate; and at
least two of the coded masks are configured to rotate relative to
one another; wherein the imager is configured to make compressed
sensing measurements of radiation from one or more radiation
sources and to generate radiation image data from the compressed
sensing measurements.
23. A radiation imager as claimed in claim 21, wherein the at least
one radiation sensor comprises: i) at least one gamma-ray radiation
sensor, such that the radiation imager constitutes a gamma-ray
radiation imager; ii) at least one neutron sensor, such that the
radiation imager constitutes a neutron radiation imager; iii) at
least one gamma-ray radiation sensor and at least one neutron
radiation sensor, such that the radiation imager constitutes a
gamma-ray radiation and neutron radiation imager; iv) at least one
dual modality sensor; or v) at least one dual modality sensor
senses both gamma-rays and neutrons.
24. A radiation imager as claimed in claim 22, further configured
to capture an optical, infrared or other wavelength image and to
output image data.
25. A radiation imager as claimed in claim 22, wherein the imager
is configured to overlay the radiation image data and an optical or
infrared image corresponding to a common field of view.
26. The radiation imager according to claim 22, wherein the coded
masks are nested.
27. A compressed sensing radiation imager as claimed in claim 22,
wherein each of the one or more masks are arcuate, cylindrical,
hemispherical, segments of spheres, or spherical.
Description
This patent application is a National Phase application of
International Application No. PCT/AU2015/000302, filed May 22,
2015, and claims priority to Australian Patent Application No.
2014901905 filed May 22, 2014. Each of the aforementioned
applications is incorporated by reference in its entirety.
FIELD OF THE INVENTION
The invention pertains to radiation detection and more particularly
to a compressed sensing gamma-ray or neutron imaging device using a
single detector and coded masks.
BACKGROUND OF THE INVENTION
Gamma-ray imaging is an important radiation detection capability
that can provide the location and identity of gamma-ray emitting
radionuclides. Gamma-ray imaging can be utilised in many
applications, including but not limited to: decommissioning,
decontamination, environmental monitoring (i.e. site surveys,
mining surveys), medical imaging (SPECT), astronomy and national
security applications (i.e. search for illicit radiological &
nuclear material).
Traditional gamma-ray imaging techniques rely on either focusing an
image onto very expensive arrays of detectors or slowly raster
scanning a single detector across the image plane. The expense of
pixelated detector arrays or slow speeds of raster scanning systems
are often prohibitive. Unlike optical photons, which are easily
focused, the highly penetrating nature of gamma-ray photons make
them very difficult to focus. Gamma-ray imaging systems that use
pixelated detector arrays typically use a single pinhole, multiple
pinhole or planar coded aperture optics. These systems are used to
form an image or an encoded image on the detector array. The use of
pinhole and coded aperture optics has been around for decades in
astronomy and medical applications. The fields of view of these
types of imaging systems are approximately 30.degree.-40.degree. in
the horizontal or vertical direction.
Rotating Modulation Collimators (RMC's), first introduced by Mertz
in 1967, typically use two masks with parallel slits that run the
entire length of the mask. When the masks are rotated, the
projection of the front mask appears to orbit the rear mask with
respect to the source. The rotation of the masks creates a
modulated count pattern at the detector that depends on the number
of sources, source intensity, location and size. The RMC has a
number of draw backs, including: a single RMC has difficulty
imaging extended sources, it has a small field of view, when using
a single RMC it is impossible to distinguish a source on the
central axis of rotation. See, B. R. Kowash, A Rotating Modulation
Imager for the Orphan Source Search Problem, PhD Thesis, 2008
The scenes to be imaged in many gamma-ray imaging applications are
sparse in nature and typically require the detection of one or more
point sources. For the simple case of a single point source that
will be sampled into a 16.times.16 image, and assuming background
is zero, this will provide 1 non-zero pixel and 255 zero pixels.
Rather than taking N (in this case 256) measurements, most of which
will be zero, intuition says that smarter strategies should be able
to determine the location of the non-zero pixel in far fewer than N
measurements. This intuition has recently been proven through the
development of a new signal processing theory, known as Compressed
Sensing. Compressed sensing is enabling new approaches to image
formation. The Compressed Sensing approach can produce images with
a fraction of the measurements (when compared to traditional
imaging techniques) and enables low cost (single detector) system
options to be realised. Single pixel imaging systems, based on
compressed sensing, have been recently developed for optical,
infra-red and THz wavelengths. See, R. G. Baraniuk et al, Method
and Apparatus for Compressive Imaging Device, U.S. Pat. No.
8,199,244 B2, 2012.
For example, a terahertz imaging system is known that uses a single
pixel detector in combination with a series of random masks to
enable high-speed image acquisition. W. L. Chan et al, A
Single-Pixel Terahertz Imaging System Based on Compressed Sensing,
Applied Physics Letters, Vol. 93, 2008. These single pixel imaging
systems all use some sort of lens to focus an image and then use
random compressive measurements to sample the image plane. However,
it should be possible to perform compressive measurements when
sampling the scene plane rather than forming an image and then
sampling. Huang et al have taken this approach and describe a
single pixel optical imaging system that requires no lens. They use
an aperture assembly to randomly sample the scene and at no stage
form a `traditional` image. G. Huang et al, Lensless Imaging by
Compressive Sensing, 2013.
The present invention overcomes shortcomings of the prior gamma-ray
imaging approaches by designing a system around the principles of
compressed sensing.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a gamma-ray imaging
device that takes fewer measurements than prior gamma-ray imaging
techniques. Images of a scene can be produced with fewer
measurements than the number of pixels in the image.
It is another object of the invention to provide a gamma-ray
imaging device having a larger field of view than prior aperture
based gamma-ray imaging techniques.
It is an object of the invention to provide a mask apparatus that
can randomly sample a scene for gamma-rays. These random
projections of the scene can be used to reconstruct images.
Accordingly, there is provided an imaging apparatus comprising a
single detector surrounded by one or more rotating masks.
In preferred embodiments, the masks are cylindrical, hemispherical,
or segments of spheres, or spheres.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
In order that the invention be better understood, reference is now
made to the following drawing figures in which:
FIG. 1 is a schematic diagram of a single detector, mask and 270
degree shield.
FIG. 2 is a schematic diagram of the single detector, mask and
shield of FIG. 1, showing additional top and bottom shields
FIG. 3 is a schematic diagram of a single detector and two nested,
rotating cylindrical masks.
FIG. 4 is a schematic diagram of a single detector and two
concentric masks showing alignment and tapering of apertures.
FIG. 5 is a schematic diagram two concentric masks showing moving
slots as an aperture system.
FIG. 6 is a schematic diagram of a mask having floating elements
bonded to a substrate.
FIG. 7 is a schematic diagram of a single detector and two
concentric hemispherical masks above a common plane.
FIGS. 8 and 9 are schematic diagrams of nested spherical masks.
FIG. 10 is a flow chart illustrating a method of operation of the
invention.
FIG. 11 is a schematic diagram of a coded mask with separate
gamma-ray and neutron blocking elements.
BEST MODE AND OTHER EMBODIMENTS OF THE INVENTION
Imager Layout and Sensing
As shown in FIGS. 1 and 2, a single gamma-ray detector 10 is
located at the centre of a mask 11 that encircles or encloses the
detector 10. The detector is located centrally of the mask or masks
preferably the detector occupies a centre or axis or rotation of
the mask 11. A cylindrical or spherical mask 11 may be used.
Although a non-central detector position can be used, it will have
a slightly different field of view. More than one detector 12, 13
can be used and these additional detectors can be in different
positions. Using multiple detectors can reduce the imaging
time.
An optional cylindrical or other radiation shield 14 may have an
arcuate opening 15 for limiting the field of view to an arc defined
by the opening 15. The mask 11 may be indexed or rotated by a
stepper motor driven turntable 19 or directly geared stepper motor
20 or otherwise to suit the coded mask or optic methodology being
employed. Through the use of stepper motors 20, gearing 21 and a
control computer 22 having for example display and print
capabilities for generating an image from the collected and
processed data, the data collection and coordinated motion/rotation
of the mask can be automated. The motion of the mask may be in
discrete steps or in a continuous movement.
As shown in FIG. 2, when a cylindrical mask 11 is used, the top and
bottom usually need to be covered by a shield 16, 17, so that the
only radiation reaching the detector is through the open apertures
18 of the mask 11 that are not otherwise shielded.
The compressed sensing gamma-ray imager may be used in conjunction
with any gamma-ray sensitive sensor 10, 12, 13. The typical
gamma-ray detector systems based on materials such as Sodium Iodide
(NaI), Caesium Iodide (CsI), Bismuth Germanate (BGO), Cadmium
Telluride (CdTe), Cadmium Zinc Telluride (CZT), High Purity
Germanium (HPGe), Strontium Iodide (SrI.sub.2) and CLYC may be
used. Spectroscopic detectors that determine the energy of each
measured photon can be used to identify the radionuclide being
imaged. Non-spectroscopic detectors that just record gross counts
will provide general information on radiation hotspots. Other
radiation detection equipment, such as dose rate meters, could be
used as the sensor and in this case would map the dose in the field
of view.
The preferred embodiment uses a spectroscopic detector that
measures the energy of each gamma-ray photon detected. The photon
count values from any particular energy bin or energy bin range can
be used as the observed data from a set of measurements. The
reconstruction of observed photon count data for a given peak
region of interest (e.g. the 60 keV .sup.241Am line) will provide
the location of the .sup.241Am, provided the radionuclide is
present. The reconstruction of observed photon data for additional
regions of interest can give the location of additional
radionuclides.
A compressed sensing neutron imager may be used in conjunction with
any neutron sensitive sensor or sensors 10, 12, 13.
Dual modality sensors 10, 12, 13, including but not limited to
CLYC, may be used to measure the modulation of both the gamma-rays
and neutrons.
It will be appreciated that the teachings of this invention may be
applied to radiation of any wavelength (or of any particle) by
using the appropriate mask and detector.
Mask and Mask Apertures
Mask pattern openings or apertures are preferably arranged in rows
and columns. The location of mask pattern openings 18 may, for
example, be produced randomly. For example, in a 16.times.16
possible aperture mask there are a total of 256 numbered apertures.
A random number generator is used to randomly select 128 of the
aperture numbers between 1 and 256. These 128 numbers are then set
to be the open apertures. The remaining 128 locations (from the
original 256 numbers) are set as zero (closed). This provides a
mask pattern that is 50% open. For rotational masks, where the mask
columns are indexed or rotated, the random selection of open/closed
apertures may be made for each row rather than the whole mask. This
would ensure that each mask row is 50% (for example) open and would
prevent situations where a row has too many or too few open
apertures, which may impact on the image reconstruction.
The geometry of the system will define the spatial resolution. The
aperture size should preferably be equal to or greater than the
detector dimensions. For example, a system may have apertures 18
with dimensions of 0.5 cm.times.0.5 cm and the cross-sectional area
of the detector should also be 0.5 cm.times.0.5 cm or less. The
further away the detector is from the mask, then the better the
spatial resolution.
Detectors with dimensions larger than those of the aperture may be
used, however, for this case there will be an increased overlap
between the fields of view of adjacent apertures. This overlap
(which is a degradation/blurriness in the spatial resolution) can
be removed by deconvolving the response function of the mask.
The preferred aperture cross-sectional shape is square. The
preferred number of apertures is a power of 2 (i.e. 64, 128, 256,
512, 1024), although it is not essential. It is preferred that
there be minimal or no separation between the mask apertures.
The thickness of the mask will depend on the application. For the
imaging of high energy photons (for example the 1.3 MeV photons
from .sup.60Co) a total mask thickness of 2 cm of lead would
attenuate approximately 72% of the 1.3 MeV photons.
The mask materials are made from a body material that can
sufficiently modulate the intensity of the incoming radiation. For
high energy gamma-rays the materials will typically be high in
atomic number (Z) and high in density, which would absorb
(attenuate) the gamma-ray radiation. Typical materials could
include but not be limited to tungsten, lead, gold, tantalum,
hafnium and their alloys or composites (i.e. 3D printing--mixing
tungsten powder with epoxy). For low energy gamma-ray photons, low
to medium Z materials, such as steel, are sufficient to modulate
the photon intensity. In a preferred embodiment the mask material
will attenuate the photons in order to modulate the photon
intensity. Other embodiments may use other interaction mechanisms,
such as Compton scattering, if they show an appreciable modulation
in photon intensity.
For imaging of neutron radiation, the mask body will need to
modulate the neutron intensity and therefore mask materials will
require a high neutron interaction cross-section. Neutron mask body
materials may include but not be limited to: Hafnium, Gadolinium,
Cadmium, Boron doped materials, Hydrogen rich materials and their
combinations.
Masks may be designed from materials that would enable the
modulation of both gamma-rays and neutrons. A single material such
as Hafnium may be suitable to modulate the intensity of both
gamma-rays and neutrons. Use of multiple materials, for example, a
combination of Tungsten and Cadmium, may be suitable to modulate
the intensities of both gamma-rays and neutrons. The open
apertures, for the gamma-ray mask, may consist of some hydrogen
rich material which does not influence the modulation of the
gamma-ray intensity. These hydrogen rich apertures would then
represent the closed apertures or modulating regions for the
neutron mask. By extension, these mask materials could be used to
modulate the intensity of any EM wavelength (i.e. optical,
infrared, THz etc) or any particle (i.e. electrons, protons
etc).
As shown in Figure 11, a coded mask is capable of modulating both
gamma-rays and neutrons separately, that is, some mask regions
being used to block gamma-rays only and some mask regions being
used to block neutrons only. In the example of FIG. 11, one sub-set
of mask regions 91 (represented in solid black) are fabricated from
a material that modulates gamma-rays only. Another sub-set of mask
regions 92 (represented in white) modulates only neutron and not
gamma-rays. Masks of this type may be fabricated in accordance with
any of the techniques and materials, shapes or configurations
disclosed by or suggested by this specification.
Masks may be singular or multiple and nested, rectangular,
circular, arcuate, hemispherical or spherical. Consecutive
measurements required for coded mask sensing will require a new
mask pattern obtained by replacing a current mask with a new one or
using some form of rotation of the mask or masks. Flat mask shapes
will have a limited field of view as they are only looking in the
forward direction, with the field of view angle determined by the
detector and mask geometry.
The advantage of arcuate, cylindrical or spherical masks is that
large fields of view (FOV) are possible. Current commercially
available pinhole/coded aperture gamma-ray cameras have horizontal
and vertical FOV between approximately 30.degree. and 40.degree..
An upright cylindrical mask embodiment would have a horizontal FOV
of 360.degree., a hemispherical mask embodiment would have a 2.pi.
FOV and a spherical mask embodiment would have a near 4.pi. FOV.
Other embodiments may include but not be limited to: ellipsoid,
cone, cuboid or hexagonal shaped masks.
In the case of a single cylindrical mask embodiment, the rotation
of the mask by one column would constitute a new mask pattern
viewing the desired FOV for a new measurement. For a single
cylindrical mask embodiment, a radiation shield can be used to
restrict the FOV and therefore have a large number of columns to
enable more measurements (see FIG. 2). The down side to the single
cylindrical mask approach is that more columns are required to
perform more measurements, which increases the diameter of the
cylinder and the physical size of the whole system.
As shown in FIG. 3, an approach utilising a nested or mask within a
mask (or dual or multiple mask approach), where each mask body 35,
36 can move or be indexed by the computer 22 independently, enables
far more measurements from the number of possible combinations of
the two mask patterns. In a preferred embodiment the dual mask
approach would consist of a cylinder within a cylinder (see FIG.
3). Each mask is rotated independently in the manner suggested for
a single mask in FIG. 2 about a sensing axis or imaging axis along
which a detector may be located. The large number of mask patterns
(and therefore measurements) would allow for a more compact system
(less total columns in one cylinder) that could image a 360.degree.
FOV. A similar argument for dual hemispherical and spherical mask
designs can also be made. For the dual mask approach, the combined
open fraction of the mask may approximate 50%, but there will be a
variation in this as the masks are rotated. One mask may be indexed
in rotation angle for a full revolution before the other mask is
indexed by a single column, thus generating a number of virtual
masks, being the number of columns squared. In other embodiments
the masks are counter-rotated by one column in an alternating or
non-alternating arrangement. Each virtual mask is used for a
radiation measurement before the next mask is generated. Each mask
need only rotate in one direction.
The cross-sectional or projected shape of the mask apertures may
include but not be limited to: square, rectangular, circular,
triangular and hexagonal. There may or may not be separation
between the mask apertures. In a preferred embodiment of a single
mask system, the mask aperture shape is square.
As shown in FIG. 4, for a dual mask embodiment the dimensions and
orientation of the inner 30 and outer mask 31 may be different,
such that they are tapered 32 (but aligned as to their edges) to
produce the same FOV for both the inner and outer masks relative to
the detector 33. The 3 dimensional shapes of these apertures 34 may
include but not be limited to a trapezoidal prism and a cone.
As shown in FIG. 5, the open apertures may be formed through the
overlapping of continuous open structures, in the form of spiral
lines 41 or some other structure on one mask and another shape such
as a vertical slit 43 on the other mask. Rotation of the masks 42,
44 relative to one another produces a coded aperture.
The mask pattern may be random, pseudo-random, non-random or
deterministic in design. The mask pattern will typically be
required to meet the defined conditions for compressed sensing to
work. A representation of the mask pattern, in matrix form, will be
used in the reconstruction process. The sensing matrix used in the
reconstruction may be a Circulant or Toeplitz matrix, which may
provide a faster computational time. In a preferred embodiment a
pseudo-random mask pattern is generated where each mask element has
an equal probability to be either 1 (open--100% transmission) or 0
(closed--0% transmission). The percentage transmission for a closed
mask element should be some value less than 100%, for example,
preferably 0% but a transmission of 50% will still be enough to
effectively modulate the intensity to reconstruct an image. The
percentage transmission relates to the increased penetrating nature
of higher energy gamma-rays. For example, a closed mask element
consisting of 10 mm lead may have 0% transmission for 60 keV
gamma-ray photons, but its percentage transmission may be
approximately 53% for 1332 keV gamma-ray photons. There will be a
point where the transmission percentages for the open and closed
apertures are too close together to modulate the photon intensity
enough to reconstruct an image. As an example, transmission
percentages of 100% and 90%, for open and closed apertures
respectively, may be too close together for sufficient modulation
in the photon intensity. There may be more than two levels of
transmission within the mask for a given energy, for example, three
levels of transmission may be 33%, 66% and 100%. Other levels of
transmission may be 25%, 50%, 75% and 100% or 0.16%, 4% and 100%.
In the latter example, the proximity of the two lower transmissions
states will effectively cause the three levels of transmission to
resemble two levels, potentially providing quicker reconstruction
times, higher quality reconstruction and few measurements. The
levels of transmission may cover two or more levels between 0% and
100%. The sensing matrix values may be the attenuation values for
particular gamma-ray energies. Different attenuation values and
therefore different sensing matrices may be used for
reconstructions at different gamma-ray energies.
As shown in FIG. 6, the mask pattern for any shape mask may be
generated such that mask structure is self-supporting. For example,
mask patterns with an array of floating or unattached "closed"
elements 50 are fixed, adhered or attached to a non-masking
substrate 51. Thus the radiation opaque mask elements 50 need not
be attached to one another other than by the substrate 51.
Alternatively, mask patterns with no floating or unattached
"closed" elements 50 may be selected, which would not require a
substrate 51, but would require the outer closed elements 50 to be
attached to a common structure.
As shown in FIGS. 7-9, the mask or masks may be hemispherical,
spherical or a part of a sphere such as a cap above any given
secant plane or optionally a segment between two planes. FIG. 7
shows two nested and concentric masks in the shape of spherical
caps, an inner cap 61 and an outer cap 62, both being hemispheres
with the rims (or lowest rows) of both in a common plane. One or
both masks 61, 62 are rotated into data sampling positions wherein
the columns 63, 64 and the rows of both are aligned or in registry
when data is sampled or acquired. Both have the same number of
columns and rows. Each row occupies a zone of a sphere between two
parallel planes. In one example, the inner hemispherical mask 61 is
indexed by one column in one direction and the outer mask 62 is
indexed or rotated by an angle defined by a single column in the
opposite direction, consistent with FIG. 3. Having both masks move
simultaneously offers greater variability in which mask elements
are open or closed when compared to having one mask stationary and
the other mask moving. This arrangement allows for single detector
coded mask imaging of the entire space above the plane that
includes the rims 65, 66.
FIGS. 8 and 9 illustrate the use of two masks or optionally two
pairs of nested masks 71, 72 that are spherical and concentric. In
this way, all of the space around the central detector or detectors
can be imaged. Each spherical mask or mask pairing 71, 72 may be
formed from 2 hemispherical masks or mask pairings as shown in FIG.
7. Each mask in the arrangement will have its own drive system
comprising a turntable or stepper motor arrangement, driven by the
system's computer 22 (See FIG. 2).
Mask Geometrical Design
The mask design will be dictated by the requirements of the
radiological imaging application in question. The geometry of the
system will influence the system performance such as spatial
resolution, FOV and sensitivity. The geometrical parameters of
importance include: the detector dimensions, the detector to mask
distance, the aperture dimensions (i.e. thickness, length and
width), the mask to source distance, the septal thickness, the
number of mask apertures and the angle subtended from the centre of
the detector and two neighbouring mask apertures. For example, a
smaller mask aperture will provide a higher spatial resolution.
Reconstruction Algorithm
There are a large number of reconstruction algorithms that have
been used for compressed sensing. For example, there are gradient
projection methods, iterative shrinkage/thresholding methods and
matching pursuit methods. See, R. M. Willett, R. F. Marcia and J.
M. Nichols, Compressed Sensing for Practical Optical Imaging
Systems: a Tutorial, Optical Engineering Vol. 50(7), July 2011. Any
of these methods or some other appropriate method can be used for
reconstructing the compressed sensing measurements. The ANSTO
compressed sensing implementation used the Gradient Projection for
Sparse Reconstruction (GPSR) algorithm. See, Gradient Projection
for Sparse Reconstruction: Application to Compressed Sensing and
Other Inverse Problems, by M. A. Figueiredo, R. D. Nowak, S. J.
Wright, Journal of Selected Topics in Signal Processing, December
2007.
Image Fusion
The gamma-ray image that is generated after the compressed sensing
measurements may be overlayed with an optical image that is
registered to the same field of view. The neutron image may be
overlayed with an optical image. The overlayed radiation images
with an optical image will help the user to visualise the location
of the radiation sources. The radiation images may be overlayed
with images at any other wavelengths (i.e. infrared).
Method
As shown in FIG. 10, a source emits radiation 80. That radiation 80
passes through a mask or masks 81 as previously disclosed. The
system's computer 22 causes the detector 10 to operate or takes a
reading from an operating detector 82. The detector then transmits
a measured value 83 to the computer 22. The computer saves and uses
the value and the positioning of the mask or masks to compile data
that will be reconstructed into an image. The computer then causes
the motor or motors controlling the mask or masks to rotate or
index to the next measurement position. Radiation then passes
through, in effect, a new mask or mask orientation 81 as the
process is repeated.
Although the invention has been described with reference to
specific examples, it will be appreciated by those skilled in the
art that the invention may be embodied in many other forms.
As used herein, unless otherwise specified the use of the ordinal
adjectives "first", "second", "third", etc., to describe a common
object, merely indicate that different instances of like objects
are being referred to, and are not intended to imply that the
objects so described must be in a given sequence, either
temporally, spatially, in ranking, or in any other manner.
Reference throughout this specification to "one embodiment" or "an
embodiment" or "example" means that a particular feature, structure
or characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment" or "in an example"
in various places throughout this specification are not necessarily
all referring to the same embodiment or example, but may.
Furthermore, the particular features, structures or characteristics
may be combined in any suitable manner, as would be apparent to one
of ordinary skill in the art from this disclosure, in one or more
embodiments.
Similarly it should be appreciated that in the above description of
exemplary embodiments of the invention, various features of the
invention are sometimes grouped together in a single embodiment,
figure, or description thereof for the purpose of streamlining the
disclosure and aiding in the understanding of one or more of the
various inventive aspects. This method of disclosure, however, is
not to be interpreted as reflecting an intention that the claimed
invention requires more features than are expressly recited in each
claim. Rather, as the following claims reflect, inventive aspects
lie in less than all features of a single foregoing disclosed
embodiment. Any claims following the Detailed Description are
hereby expressly incorporated into this Detailed Description, with
each claim standing on its own as a separate embodiment of this
invention.
Unless specifically stated otherwise, as apparent from the
following discussions, it is appreciated that throughout the
specification discussions utilizing terms such as "processing,"
"computing," "calculating," "determining" or the like, refer to the
action and/or processes of a microprocessor, controller or
computing system, or similar electronic computing or signal
processing devices, that manipulates and/or transforms data.
Furthermore, while some embodiments described herein include some
but not other features included in other embodiments, combinations
of features of different embodiments are meant to be within the
scope of the invention, and form different embodiments, as would be
understood by those in the art. For example, in the following
claims, any of the claimed embodiments can be used in any
combination.
Thus, while there has been described what are believed to be the
preferred embodiments of the invention, those skilled in the art
will recognize that other and further modifications may be made
thereto without departing from the spirit of the invention, and it
is intended to claim all such changes and modifications as fall
within the scope of the invention.
While the present invention has been disclosed with reference to
particular details of construction, these should be understood as
having been provided by way of example and not as limitations to
the scope or spirit of the invention.
* * * * *
References